You need enough positive and negative wires to safely meet the boards power requirements. People are combining multiple black/yellow wires in parallel because typically single cables from an ATX PSU aren't of a sufficient gauge to carry the current that the board demands. Too much current down one wire means the wire gets hot and.. perhaps bad times.

I don't understand why the OP has 4 black and 2 yellow, there's no benefit to running additional wires on only one side of the supply.

One of the first round auction winners had an 'issue' when spreading inputs across the two terminals, see http://www.youtube.com/watch?v=cvAFBWHGw58.

While good intentioned and with some useful information regarding vertical mounting and cooling, there's a lot of misinformation around how to most safely power these boards across this thread and others. Detail the specifics of your proposed configuration and ask questions if in doubt. Above all, be safe - wires feeling too "warm" (think would you feel safe with a mains extension lead at that temperature) may suggest something that isn't suitable to be run 24/7 for months on end as these blades inevitably will be.

I didn't intend to scaremonger, honest!